High velocity jets of water in the form of discrete droplets, or with cavitation bubbles therein are being used increasingly for surface cleaning operations. Such operations include the removal of paint or other protective film from roads, structures, stone or brick facades, removing grease, clinker or chemical products such as rust from tanks, pipes heat exchangers or the like. It is also often necessary to clean badly corroded metal surfaces to a white metal finish, or to remove barnacles and marine growth from ships hulls, tower legs or the like that are normally under water.
With nozzles that use discrete water droplets, a fairly large stand-off distance is used, i.e., the distance between the nozzle orifice and the work surface. The physical impact of such droplets on a target surface causes a comples pattern of intense transient stresses. These stresses cause break-down and removal of surface material. For removing the most resistant materials, the use of vapour filled cavities, i.e. cavitation bubbles was proposed. The collapse of vapour-filled cavities also generates intense transient stresses which can be caused to remove surface material. The collapse of these cavities has the potential, for a given jet velocity, of generating stresses even higher than those obtainable by droplet impact.
To date, cavitation nozzles have had limited commercial development. Further, researchers in this art have had conflicting views as to the superiority of either form of nozzle over the other. Here, the reader is referred to papers such as:
(a) Conn, A. F., Rudy, S. L., and Mehta, G. D., "Development of a cavijet system for removing marine fouling and rust," Proc. 3rd International Symposium on Jet Cutting Technology: Paper G4, organized by Brit. Hydromech. Res. Assoc., Chicago, U.S.A., May 11-13, 1976.
(b) Beutin, E. G., Erdmann-Jesnitzer, F., and Louis, H., "Influence of cavitation bubbles in cutting jets," Proc. 2nd International Symposium on Jet Cutting Technology: Paper D3, organised by Brit. Hydromech. Res. Assoc., Cambridge, Apr. 2-4, 1974.
(c) Lichtarowicz, A., "Experiments with cavitating jets," Proc. 2nd International Symposium on Jet Cutting Technology: Paper D11, Apr. 2-7, 1974.
(d) Thiruvengadam, A., "The concept of erosion strength," Erosion by Cavitation or Impingement, ASTM STP 408, Am. Soc. Testing Mats., 1967 pg. 22.
(e) Hammitt, F. G., "Collapsing bubble damage to solids," Cavitation state of knowledge, ASME, 87-102, 1969.
A reader is also directed to Canadian and U.S. Pat. Nos. 967,940 of May 20, 1975 and 3,713,699, respectively, which issued to Hydronautics Incorporated (Virgil E. Johnson, Jr.), and U.S. Pat. No. 3,572,839 which issued in March, 1971 to Okabe. These patents show prior art constructions which embody certain advantageous features. The Johnson patents, for example, describe some embodiments of a cavitation nozzle that "submerges" the high velocity jet in a liquid while effecting a cleaning operation on a surface. That surface may itself be actually submerged. Alternatively, the high velocity jet is artificially "submerged" by being surrounded by a shroud of the same liquid at low pressure and substantially stationary as compared to the high velocity jet.